This invention relates to measuring corrosion and it relates more particularly to instruments and techniques used in the study of corrosion processes, and most particularly to the monitoring of corrosion rates on a number of different metals and for various galvanic situations, preferably under actual process conditions.
Industrial plants, as well as oil field equipment, are built from many different structural and functional metals, and often a multitude of these are in contact with the aggressive fluids. It has been assumed in the past that the corrosion rates of these different metals (in particular, the different steels) can be adequately represented by a single test specimen such as AISI-1018 steel. This assumption is incorrect for several reasons:
(a) It has been observed that pitting rates on these different steels are entirely different. PA1 (b) The response to inhibitors of different metals is often quite different. For instance, it has been observed that the corrosion of SA-533A steel increases much faster at marginal corrosion inhibitor concentrations than the corrosion rate for AISI-1018 or ASTM-285C. PA1 (c) Different metals respond differently to process upsets (in chemical cleaning studies it has been found, for instance, that ASTM-533A alloy corrosion is accelerated much less in the presence of oxygen or under oxidizing conditions than AISI-1018 carbon steel.) PA1 (d) Galvanic coupling of metals is unavoidable and different metals again respond differently to such effects.
It is therefore, essential to monitor corrosion rates on a number of different metals and for various galvanic situations as well as under actual process conditions. Such monitoring may be concerned with general corrosion rates, pitting corrosion, adequate protection by inhibitors or for the purpose of determining process upsets. The purpose of such monitoring, of course, is aimed at failure prediction and proper maintenance outage scheduling.
Corrosion rates can be conveniently monitored by the linear polarization technique such as is carried out with the PAIR instruments and probes available from the Petreco Division of the Petrolite Corporation. A PAIR instrument is described and claimed in U.S. Pat. No. 3,406,101, issued Oct. 15, 1968 to James W. Kilpatrick, the disclosure of which is hereby incorporated by reference. The PAIR technique employs three electrodes on an electrode holder, of which one is the test electrode (cathode), another one used as auxiliary electrode (anode) and the third as reference electrode. This technique, therefore, requires one entry port in order to attain one data point. Furthermore, it has been assumed in the past that the linear polarization technique is only adequate for general corrosion rate monitoring while a zero resistance ammeter is necessary to monitor galvanic current situations. Therefore, the current state of the art requires several instruments and a multitude of entry ports into an actual system in order to monitor corrosion on different metals and for galvanic situations.
The objects of this invention are to provide instrumentation, circuitry and a method to enable this complex monitoring task to be simplified.
Other objects of the invention will be apparent from the following description of the invention.